A thin film solar cell may employ a CIS thin film or a CIGS thin film as a photo-absorption layer. The CIS thin film or CIGS thin film is a Group I-III-VI compound semiconductor. Specifically, the CIS or CIGS thin film may be formed to a thickness of 10 micrometers or less and exhibits stable properties in long term use. Thus, the thin film is expected to be used as a low cost and high efficiency solar cell capable of replacing a silicon thin film.
Specifically, the CIS thin film is a direct transition type semiconductor and may be formed into a thin film. Further, the CIS thin film has a band-gap of 1.04 eV, which is suitable for photo conversion, and exhibits the highest coefficient of photo absorption among solar cell materials known in the art. In addition, the CIGS thin film is developed to replace some of indium (In) by gallium (Ga) or to replace sulfur (S) by selenium (Se) to improve low open circuit voltage of the CIS thin film.
A CIGS solar cell is manufactured using a thin film having a thickness of several micrometers by vacuum deposition or non-vacuum coating. Vacuum deposition has an advantage in that it provides a highly efficient absorption layer. However, vacuum deposition provides low uniformity when a large area absorption layer is formed, and requires high manufacturing costs.
Among vacuum deposition processes, a three-step vacuum co-evaporation process widely known in the art will be described hereinafter.
FIG. 8 depicts a thermal history curve in a typical three-step vacuum co-evaporation process. FIG. 9 is a flowchart of the typical three-step vacuum co-evaporation process.
Referring to FIGS. 8 and 9, the typical three-step vacuum co-evaporation process is largely comprised of three steps. Particularly, in Step 1, In, Ga, and Se are co-evaporated on a substrate on which a Mo electrode is formed, thereby forming a thin film on the substrate. In Step 2, Cu and Se are co-evaporated at high temperature. In Step 3, In, Ga, and Se are co-evaporated again to form a CIS-based thin film.
Particularly, in Step 2, due to co-evaporation of Cu and Se, the two component Cu—Se compound acts as a flux at high reaction temperature and fills the grain boundary of previously formed (In, Ga)2Se3 particles to grow particles, which in turn fill voids to form a CIGS thin film having a dense structure. This is one important feature of vacuum co-evaporation as a vacuum deposition technique.
On the other hand, heat treatment at high temperature after depositing a precursor material may reduce manufacturing costs and is capable of preparing a large scale layer uniformly. However, heat treatment has a drawback in that it provides low efficiency of an absorption layer.
Since the CIGS thin film formed by depositing a precursor material in a non-vacuum state has lots of pores and exhibits non-dense characteristics, selenization heat treatment is performed. Since typical selenization heat treatment is performed using toxic hydrogen selenide (H2Se) gas, high installation costs are required to provide a safety system to guarantee safety. Further, since heat treatment is performed for a long period of time, there is a drawback that manufacturing costs for the CIGS thin film increase.
In addition, since the CIGS thin film has very high melting point of 1000° C. or more, it is difficult even for CIGS particles having a size of dozens of nanometers to allow particle growth and densification through post-heat treatment.
As the related art, there are Korean Patent Publication No. 10-2009-0043265A, Korean Patent Publication No. 10-2007-0055497A, and the like.